The present invention relates generally to a knocking control method and an apparatus for detecting and suppressing knocking in an internal combustion engine (hereinafter also referred to simply as the engine) such as a gasoline engine for a motor vehicle. More particularly, the invention is concerned with improvements in a knocking control method and an apparatus which can improve controllability of engine operation while assuring reduction in the cost involved in implementation of control hardware.
In general, internal combustion engines such as gasoline engines for motor vehicles include a plurality of cylinders in each of which an air fuel mixture is compressed and combusted at an optimal ignition timing. In this conjunction, there has already been proposed and widely used in practical applications a microcomputer- or microprocessor-based engine control unit (also known as an ECU in abbreviation) for the purpose of optimally controlling the ignition timing as well as the sequence of fuel injections in the individual engine cylinders.
In connection with such engine control, it is known that when the ignition timing (usually given in terms of angular crank position) is controlled to advance excessively, abnormal fuel combustion can take place, resulting in the generation of severe vibrations or shock of the engine cylinder (referred to as knocking) of such a magnitude which may eventually damage or injure the engine. In order to avoid such an unwanted phenomenon, it is necessary to control the ignition timing such that upon detection of abnormal vibrations, the ignition timing is shifted in the direction to afford an appropriate retard to the time point or timing at which the fuel combustion takes place in the engine cylinder.
For a better understanding of the background of the invention, an engine knocking control apparatus known heretofore will be described in some detail with reference to FIG. 6, which is a block diagram showing the general arrangement of the known knocking control apparatus.
In FIG. 6, a reference numeral 1 denotes a knocking sensor installed in association with one or each of the cylinders of an internal combustion engine. The knocking sensor 1 can be constituted by a piezoelectric element or the like component which is capable of detecting knocking in the form of vibrations of the associated cylinder as an electric signal.
The output signal A of the knocking sensor 1 is supplied to a knocking detection circuit, which is generally denoted by a reference numeral 2. The knocking detection circuit 2 is composed of a filter 21 having such a filtering characteristic as to pass therethrough only the frequency components which are peculiar to the vibration phenomenon (e.g., 7 kHz), a gate 22 for allowing the output signal of the filter 21 to periodically pass therethrough at a predetermined timing, a background level (BGL) generator 23 for generating a background level signal BGL on the basis of an output signal A' of the gate 22, a comparator 24 for comparing the output signal A' of the gate 22 with the background level signal BGL to produce an output signal of "ON" level when the gate output level A' exceeds the background level BGL, and an integrator 25 for integrating the output signal of the comparator 24. The output signal of the integrator 25 is then supplied to an analogue to digital (A/D) converter 3 where it is converted to a digital signal V.sub.R.
The digital signal V.sub.R is supplied to an engine control unit (ECU) 4 which may be constituted by a microcomputer or microprocessor and which is programmed to perform ignition timing control for the engine cylinders on the basis of the output signal V.sub.R of the A/D converter 3 while supplying a masking pulse signal M to the gate 22 and a reset signal R to the integrator 25, respectively, for the purpose which will be described hereinafter. Further, the ECU 4 includes a controller 4A for controlling the ignition timing of the cylinders. The controller 4A calculates an optimal ignition timing for each cylinder on the basis of the operating condition of the engine under normal combustion therein in the well-known manner as well as an angle of retard for a knocking cylinder on the basis of the digital signal V.sub.R output from the A/D converter 3, so that the optimal ignition timing for the knocking cylinder is to be delayed by the angle of retard for suppressing the knocking. To this end, the controller 4A is designed to produce a control angle signal .theta.R based on the optimal ignition timing and the angle of retard for properly controlling the ignition timing and in particular for suppressing knocking when it is taking place.
Next, description will be made of the operation of the knocking control apparatus of FIG. 6, while referring to a waveform diagram shown in FIG. 7.
Normally, in each of the engine cylinders, ignition takes place at a timing corresponding to an crank angle or position which advances approximately by about 5.degree. relative to top dead center (TDC) (which is given by the crank angle of 0) so that explosive combustion of the air fuel mixture may occur at the timing corresponding to a crank angle in a range of about 10.degree. to 60.degree. after TDC. The knocking due to the abnormal combustion will thus take place at the timing falling within the crank angle range of about 10.degree. to 60.degree. after TDC.
Accordingly, upon every occurrence of vibration noise of the cylinder and inter alia knocking, the output signal A of the knocking sensor 1 produced at a corresponding periodical time interval assumes a significantly increased amplitude, as can be seen in the waveform shown in FIG. 7 at (a).
In the meanwhile, the ECU 4 outputs to the gate 22 a masking pulse signal M which is inverted periodically at a predetermined time interval in order to ensure that the knocking detection circuit 2 can receive and efficiently process the sensor output signal A. More specifically, the masking pulse signal M is generated in such a waveform in which the leading edge thereof appears at a crank angle of about 75.degree. before TDC (this advance angle will hereinafter be represented by affixing "B" to the angle value, e.g., by "B75.degree.") while the trailing edge of the masking pulse M approximately corresponds to a time point B5.degree. of 5.degree. before TDC, as can be seen in the waveform shown at (b) in FIG. 7. So long as the masking pulse M assumes a high level, the gate 22 is blocked or disabled. Further, as mentioned previously, a reset signal R is supplied to the integrator 25 from the ECU 4 periodically at a predetermined timing which coincides with the leading edge of the masking pulse signal M.
The filter 21 incorporated in the knocking detection circuit 2 has such a filtering characteristic that the frequency components of the knocking sensor output signal A produced upon occurrence of cylinder or engine vibrations can pass therethrough, while the gate 22 allows the knocking sensor output signal A to pass therethrough only during a period in which the masking pulse signal M is at a low level, as shown at (c) in FIG. 7. On the other hand, the background level generator 23 generates the background level BGL contained in the output signal A' of the gate 22 by discriminatively separating the former from the latter, as is illustrated at (d) in FIG. 7. The background level BGL thus derived serves as a reference signal or threshold for the detection of knocking.
When the gate output signal A' exceeds the background level BGL, the comparator 24 decides that knocking has taken place and produces an output signal of "H" level. The integrator 25 starts to integrate the output signal of the comparator 24 after it is reset by the reset signal R applied thereto from the ECU 4, as illustrated at (e) in FIG. 7. The output signal V.sub.R of the integrator 25 then undergoes A/D conversion by the A/D converter 3, the resulting digital value being then input to the ECU 4.
In this manner, the ECU 4 receives the A/D converted integration value V.sub.R upon every occurrence of ignition and combustion in the engine cylinder, to thereby generate a control signal .theta..sub.R in the form of a retarded control angle signal for controlling the ignition timing in the sense to suppress knocking if it is taking place. To this end, the controller 4A constituting a part of the ECU 4 adds an angle of retard .delta..theta..sub.R, which is to be described in detail later, to a current ignition control angle .theta..sub.R *, which is suitable for normal or optimal ignition, thereby to generate a current retarded control angle signal .theta..sub.R. Accordingly, the current retarded control angle .theta..sub.R can be given by the following expression: EQU .theta..sub.R =.theta..sub.R *+.delta..theta..sub.R ( 1).
In expression (1) above, the angle of retard .delta..theta..sub.R is given by the following formula: EQU .delta..theta..sub.R =V.sub.R .times.L
where L represents a weighting constant.
As will be appreciated from the foregoing, in the case of the known engine knocking control apparatus described above, knocking control is performed by resorting to the use of the knocking detection circuit 2 in combination with the knocking determining or identifying means constituted by the background level generator 23, the comparator 24, the integrator 25 and others. Under the circumstances, the known knocking control apparatus suffers from the following problems: many hardware components are required for knocking control; the entire arrangement of hardware components becomes complicated; and thus, high cost or expenditure is involved in the manufacture of the knocking control apparatus as a whole.